5 AE and Failure Characteristic
Figure 10 illustrates the evolution of AE account and accumulative AE
counts of granite specimens under cyclic loading-unloading compression,
AE and AAE represent as AE account and accumulative AE counts,
respectively. On the whole, AAE shows a ladder-like growth with duration
time. At room temperature, weak AE events are monitored at the initial
compaction and elastic stage. When the loading near the peak strength,
accumulative AE hits thrived increases. After the peak strength, axial
stress drop quickly, and violent AE events are monitored. However, the
AE events decreases obviously when the loading enter to the residual
strength stage, and few micro-crack initiated due to the friction
between the micro-crack under cyclic loading process. Compared with
Figs. 4 and 7, it can be seen that the evolution of AAE is similar to
that of axial plastic strain and dissipated energy, whereas there is
also difference. This may be because dissipated energy result in not
only the plastic strain but also crack propagation. Under uniaxial
compression, the coupling is insufficient between AE probe and specimen,
and the AAE is lesser than that under triaxial compression.
When T = 600°C, more thermal cracks are induced in the specimen,
and lead to the carrying capacity loss under uniaxial compression. Under
cyclic loading process, displacement occurred between grains with less
friction, and less AE events are monitored, as shown in Fig. 10a.
Confining pressure closed the thermal induced crack and increases
friction between grains, therefore more AE events are monitored once
applied loading. Compared with Figs. 10b and 10d, after thermal
treatment, violent AE events is lesser than that at room temperature,
whereas AAE is more than that at room temperature. It means that the
micro-crack is easier to initiate and dispersed distribute in the
specimen, and the loading process shows plastic characteristic whenT = 600°C.
Figures 11 depict the comparison of the ultimate failure model of
granite after thermal treatment between triaxial monotonic and cyclic
loading compression, it is clear that loading path has slight effect on
the ultimate failure modes. Under uniaxial compression, the granite
specimens under monotonic and cyclic loading failed with axial
splitting. Under triaxial compression, the distribution of cracks under
cyclic loading is more complicated than that under monotonic loading,
even though the specimen failed with shear cracks. On the one hand, more
energy dissipated in the granite specimens under cyclic loading, which
result in more micro-cracks initiation. Therefore, the potential
position to form macro-cracks in the specimen increases. On the other
hand, the shear cracks propagated quickly under monotonic loading after
the peak strength. However, the shear cracks propagated step by step
under cyclic loading. The shear cracks near stop propagated under
unloading process, and then it may not propagate in the original
direction under loading process. Therefore, the distribution of cracks
in the specimens under cyclic loading is more complicated than that
under monotonic loading. This phenomenon is also obtained by Yang et al.
(2015).